The control methods for controlling the power-factor-improving-type switching power supply apparatuses include a critical conduction mode control method and a continuous conduction mode control method. In the critical conduction mode control method, a switching device is turned on as the inductor current becomes 0. In the continuous conduction mode control method, the inductor current is controlled so as not to be 0.
Since the switching device conducts switching when the inductor current is 0 in the critical conduction mode control method generally, the switching noise caused is low. Therefore, the critical conduction mode control method is advantageous for reducing the noises. However, since a large current ripple is caused in the critical conduction mode control method, a large stress is exerted to the inductor and the diode. Therefore, it is hard to apply the critical conduction mode control method to the use, in which the load is heavy.
Although a larger switching noise is caused in the continuous conduction mode control than in the critical conduction mode control, a smaller current ripple is caused in the continuous conduction mode control and the stress exerted to the inductor and diode is small. Therefore, the continuous conduction mode control method is applicable to the use, in which the load is heavy. Therefore, it is general to employ the critical conduction mode control method when the load is 100 watt or lighter and to employ the continuous conduction mode control method when the load is heavier than 100 watt.
FIG. 6 is a block circuit diagram showing the conventional switching power supply apparatus of the continuous conduction-mode-control-type disclosed in Japanese Unexamined Patent Application Publication No. Hei. 04 (1992)-168975. In FIG. 6, AC power supply 1, rectifier circuit 2 formed of a diode bridge, capacitors 3 and 6, inductor 4, diode 5, switching device 7 such as a metal oxide-film field-effect transistor (hereinafter referred to as “MOSFET”), voltage error amplifier 8, multiplier 9, comparator 10, monostable multivibrator 11, resistor for current detection (hereinafter referred to as “current detecting resistor”) 12, and driver circuit 13 are shown.
The output voltage from AC power supply 1 is full-wave-rectified by rectifier circuit 2 and high frequency noises are removed by capacitor 3. A current is fed to capacitor 6 via inductor 4 and diode 5 and smoothed DC voltage Vout is outputted. The one end of switching device 7 such as a MOSFET is connected between inductor 4 and diode 5. As switching device 7 is turned on and the inductor current from inductor 4 flows through switching device 7, energy is stored in inductor 4. As switching device 7 is turned off, the inductor current is turned to flow to diode 5 and the energy stored in inductor 4 is released to the output side.
Voltage error amplifier 8 outputs error voltage Verr obtained by amplifying the difference between output voltage Vout and output reference voltage Vref to multiplier 9. Multiplier 9 multiplies error voltage Verr and input voltage Vin and generates threshold signal Vth in-phase with and similar to input voltage Vin and having an amplitude proportional to error voltage Verr.
The current flowing through inductor 4 is converted into current detection signal Vi by current detecting resistor 12 and current detection signal Vi is compared with threshold signal Vth in comparator 10. The output from comparator 10 is fed to the trigger input of monostable multivibrator 11. Monostable multivibrator 11 keeps, after the trigger signal is inputted thereto, the output therefrom at a low level for a certain period and, then, changes the output therefrom to a high-level one. The output from monostable multivibrator 11 is fed to driver circuit 13. Driver circuit 13 turns on switching device 7 when the input thereto is at a high level and turns off switching device 7 when the input thereof is at a low level.
As switching device 7 in the circuit configured as described above is turned on, the current from inductor 4 increases and current detection signal Vi rises. As current detection signal Vi becomes equal to or larger than threshold signal Vth, the output from comparator 10 is set at the high level and a trigger signal is fed to monostable multivibrator 11. As the trigger signal is fed to monostable multivibrator 11, the output from monostable multivibrator 11 is set at the low level and switching device 7 is turned off by driver circuit 13. As switching device 7 is turned off, the current from inductor 4 decreases gradually. Since the low level period of monostable multivibrator 11 is set so as not to make the current from inductor 4 decrease to 0, the output from monostable multivibrator 11 changes the level thereof to high one as the current from inductor 4 decreases to some extent and switching device 7 is turned on by driver circuit 13.
FIGS. 7A and 7B describe the operations described in the preceding paragraphs. The peaks of the detection signal indicating the current flowing through inductor 4 are controlled to coincide with threshold signal Vth in-phase with and similar to input voltage Vin. Since the ON-period changes while the OFF-period is fixed, the switching frequency changes and the frequencies of the noise caused also change. Therefore, the noise spectrum is diversified and the noises caused in the continuous conduction mode of operation are reduced. FIG. 7A is a wave chart describing the relation between threshold signal Vth and current detection signal Vi. FIG. 7B is a wave chart describing the ON- and OFF-waveform of switching device 7.
For improving the power factor, it is necessary for the input current to be in-phase with and similar to the input voltage. For setting the input current to be in-phase with and similar to the input voltage, it is necessary to change the ON-OFF duty ratio widely from the vicinity of 0% to the vicinity of 100%. The voltage across the inductor in the 100 V system is different from the voltage across the inductor in the 200 V system. Moreover, the voltage across the inductor changes during the one cycle of the AC input voltage. Therefore, the changing rate (di/dt) of the current flowing through the inductor changes greatly. Therefore, the current variation in a certain period changes greatly depending on the value and phase of the input voltage and the state of the load. If the OFF-period is fixed as in the conventional continuous conduction mode control, the right magnitude of current change will not be obtained and the power factor will not be improved as expected.
Japanese Unexamined Patent Application Publication No. 2007-143383 proposes a method for improving the power factor, which sets first threshold signal VTh1 and second threshold signal VTh2 proportional to first threshold signal VTh1 such that VTh1>VTh2, turns off the switching device as the current flowing through the inductor reaches first threshold signal VTh1, and turns on the switching device as the current flowing through the inductor reaches second threshold signal VTh2. According to the method described above, the ON- and OFF-periods are not fixed and the switching device is turned on and off automatically for the optimum periods depending on the state of the input voltage and the state of the load and, by which the power factor is improved.
The switching frequency (period) is determined by the difference between first and second threshold signals VTh1 and VTh2 and by the changing rate of the inductor current. The changing rate of the inductor current is determined by the input voltage, the output voltage and the inductor inductance. The parameters that determine the switching frequency change in various manners. First threshold signal VTh1, that is the product of error signal Verr and input voltage Vin, changes, since error signal Verr changes as the load weight changes. (Error signal Verr changes to be small under a light load and to be large under a heavy load.) The inductance changes due to the variations caused during the manufacture thereof and the temperature characteristics thereof. The input voltage changes greatly between the 100 V system and the 200 V system. The output voltage changes depending on the use of the switching power supply apparatus. The output voltage also changes a little bit depending on the load weight. Since the parameters that determine the switching frequency change as described above, the switching frequency changes in various manners.
However, as the average switching frequency increases, the maximum frequency becomes too high, increasing the noise and switching loss. If the average switching frequency decreases on the other hand, the response performance of the switching power supply apparatus will be impaired.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a switching power supply apparatus that controls second threshold signal Vth2 to keep the average switching frequency at a certain value for reducing the noises and losses so that the power factor may be improved and the response performance of the power supply may be prevented from being impaired, even if the parameters such as first reference signal Vth1, the input voltage, the output voltage and the inductance change.